hakmem/core/tiny_box_geometry.h

// tiny_box_geometry.h - Box 3: Geometry & Capacity Calculator
// Purpose: Centralize stride/capacity/base calculations for Tiny allocator
//
// Box Theory Responsibility:
// - Calculate block stride (size + header) with C7 (headerless) handling
// - Calculate slab capacity (usable bytes / stride)
// - Calculate slab base address with Slab 0 offset handling
// - Provide single source of truth for geometry calculations
//
// This Box eliminates code duplication and makes C7 special cases explicit.

#ifndef TINY_BOX_GEOMETRY_H
#define TINY_BOX_GEOMETRY_H

#include <stdint.h>
#include <stddef.h>
#include "hakmem_tiny_superslab_constants.h"
#include "hakmem_tiny_config.h"  // For g_tiny_class_sizes declaration

// ============================================================================
// Box 3 API: Geometry Calculations (Single Source of Truth)
// ============================================================================

/**
 * Calculate block stride for a given class
 *
 * Phase E1-CORRECT: ALL classes have 1-byte header (unified box structure)
 *
 * @param class_idx Class index (0-7)
 * @return Block stride in bytes (total block size)
 *
 * Box Structure: [Header 1B][User Data N-1B] = N bytes total
 * - g_tiny_class_sizes[cls] = total block size (stride) = N
 * - usable data = N - 1 (implicit)
 * - All classes follow same structure (no C7 special case!)
 */
static inline size_t tiny_stride_for_class(int class_idx) {
#if HAKMEM_TINY_HEADER_CLASSIDX
    // Phase E1-CORRECT: g_tiny_class_sizes stores TOTAL size (stride)
    // ALL classes have 1-byte header, so usable = stride - 1
    return g_tiny_class_sizes[class_idx];
#else
    // No headers: stride = usable size
    return g_tiny_class_sizes[class_idx];
#endif
}

/**
 * Calculate slab capacity (number of blocks that fit in usable space)
 *
 * @param slab_idx Slab index within SuperSlab (0 for first slab, 1+ for others)
 * @param stride Block stride in bytes (from tiny_stride_for_class)
 * @return Number of blocks that fit in this slab
 *
 * Slab 0 has reduced usable space (SUPERSLAB_SLAB0_USABLE_SIZE)
 * Slabs 1+ have full usable space (SUPERSLAB_SLAB_USABLE_SIZE)
 */
static inline uint16_t tiny_capacity_for_slab(int slab_idx, size_t stride) {
    size_t usable = (slab_idx == 0)
                        ? SUPERSLAB_SLAB0_USABLE_SIZE
                        : SUPERSLAB_SLAB_USABLE_SIZE;
    return (uint16_t)(usable / stride);
}

/**
 * Get slab base address (accounts for SUPERSLAB_SLAB0_DATA_OFFSET)
 *
 * @param ss SuperSlab pointer
 * @param slab_idx Slab index within SuperSlab
 * @return Pointer to first usable byte in this slab
 *
 * Slab 0 has an offset (SUPERSLAB_SLAB0_DATA_OFFSET) due to SuperSlab metadata
 * Slabs 1+ start at slab_idx * SLAB_SIZE
 */
static inline uint8_t* tiny_slab_base_for_geometry(struct SuperSlab* ss, int slab_idx) {
    uint8_t* base = (uint8_t*)ss + (slab_idx * SLAB_SIZE);
    // Slab 0 offset: sizeof(SuperSlab)=1088, aligned to next 1024-boundary=2048
    if (slab_idx == 0) base += SUPERSLAB_SLAB0_DATA_OFFSET;
    return base;
}

/**
 * Calculate usable bytes for a given slab
 *
 * @param slab_idx Slab index within SuperSlab
 * @return Usable bytes in this slab
 */
static inline size_t tiny_usable_bytes_for_slab(int slab_idx) {
    return (slab_idx == 0)
               ? SUPERSLAB_SLAB0_USABLE_SIZE
               : SUPERSLAB_SLAB_USABLE_SIZE;
}

/**
 * Calculate block address within a slab (linear allocation)
 *
 * @param base Slab base address (from tiny_slab_base_for_geometry)
 * @param index Block index (0-based)
 * @param stride Block stride (from tiny_stride_for_class)
 * @return Pointer to block at given index
 */
static inline void* tiny_block_at_index(uint8_t* base, uint16_t index, size_t stride) {
    return (void*)(base + ((size_t)index * stride));
}

/**
 * Validate that a linear carve operation stays within slab bounds
 *
 * @param slab_idx Slab index
 * @param carved Current carved count
 * @param stride Block stride
 * @param reserve Number of blocks to reserve
 * @return 1 if operation is safe, 0 if it would exceed bounds
 */
static inline int tiny_carve_guard(int slab_idx,
                                   uint16_t carved,
                                   size_t stride,
                                   uint32_t reserve) {
    size_t usable = tiny_usable_bytes_for_slab(slab_idx);
    size_t needed = ((size_t)carved + (size_t)reserve) * stride;
    return needed <= usable;
}

// ============================================================================
// Box 3 Debug Helpers (fail-fast validation)
// ============================================================================

/**
 * Debug helper: verbose carve guard with diagnostics
 *
 * @param stage Debug stage name
 * @param class_idx Class index
 * @param slab_idx Slab index
 * @param carved Current carved count
 * @param used Current used count
 * @param capacity Slab capacity
 * @param stride Block stride
 * @param reserve Blocks to reserve
 * @return 1 if safe, 0 if would exceed bounds (with stderr logging)
 */
static inline int tiny_carve_guard_verbose(const char* stage,
                                           int class_idx,
                                           int slab_idx,
                                           uint16_t carved,
                                           uint16_t used,
                                           uint16_t capacity,
                                           size_t stride,
                                           uint32_t reserve) {
#if HAKMEM_BUILD_RELEASE
    (void)stage; (void)class_idx; (void)slab_idx;
    (void)carved; (void)used; (void)capacity;
    return tiny_carve_guard(slab_idx, carved, stride, reserve);
#else
    size_t usable = tiny_usable_bytes_for_slab(slab_idx);
    size_t needed = ((size_t)carved + (size_t)reserve) * stride;
    if (__builtin_expect(needed > usable, 0)) {
        fprintf(stderr,
                "[LINEAR_GUARD] stage=%s cls=%d slab=%d carved=%u used=%u cap=%u "
                "stride=%zu reserve=%u needed=%zu usable=%zu\n",
                stage ? stage : "carve",
                class_idx,
                slab_idx,
                carved,
                used,
                capacity,
                stride,
                reserve,
                needed,
                usable);
        return 0;
    }
    return 1;
#endif
}

#endif // TINY_BOX_GEOMETRY_H